tostring isnt working string expected, got Object?

12:58:22.076 - ServerScriptService.SkillEvents.EquipSkillZ:11: bad argument #2 to '?' (string expected, got Object) is the error

here is the script

print(plr.PlayerGui.SkillsUi.SkillZ:WaitForChild("SkillIcon").Image)
local skimage = plr.PlayerGui.SkillsUi.SkillZ:WaitForChild("SkillIcon")
skimage.Image = image
local skillvalue = tostring(Skill) -- skillvalue is SwordSpin
plr.Buttons[Button].Value = skillvalue -- Error is here